Phase transitions in liquid crystal 6O.4 (p-<Emphasis Type="Italic">n</Emphasis>-hexyloxybenzylidine-p′-<Emphasis Type="Italic">n</Emphasis>-butylaniline)

DSC measurements on p-n-hexyloxybenzylidine-p′-n-butylaniline (6O.4) showed that the crystalline to liquid crystalline (K-S _H) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the K-S _H transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasielastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (K and S _H) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the S _H phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.      

Structural refinement,optical and ferroelectric properties of microcrystalline Ba(Zr0.05Ti0.95)O3 perovskite

For this study, a microcrystalline Ba(Zr_0.05Ti_0.95)O₃ (BZT) powder was prepared by a high energy ball milling method followed by calcination at 1100 °C for 4 h. The calcined powder was structurally characterized by X-ray diffraction and Rietveld refinement data, which showed that this material has a perovskite-type tetragonal structure with a space group of (P4mmm). The micro-Raman spectrum revealed local lattice distortions due to distorted octahedral [TiO₆] clusters. The temperature and frequency-dependent dielectric study of the BZT ceramic showed normal phase transition behavior. The ferroelectric property was studied by a P–E hysteresis loop. Optical band gap was investigated by ultraviolet–visible (UV–vis) absorption spectroscopy at room temperature. The UV–vis spectrum indicated that the BZT powder has an optical band gap of 3.15 eV.     

Influence of fabrication processing on phase transition and electrical properties of 0.8Pb(Zr1/2Ti1/2)O3–0.2Pb(Ni1/3Nb2/3)O3 ceramics

The binary system of 0.8Pb(Zr_1/2Ti_1/2)O₃–0.2Pb(Ni_1/3Nb_2/3)O₃ ceramics were synthesized by conventional mixed oxide and columbite method. X-ray diffraction analysis demonstrated the coexistence of both the rhombohedral and tetragonal phases for the columbite prepared sample. Rhombohedral to tetragonal phase transition for columbite method was different compared with those of the mixed oxide method. The permittivity shows a shoulder at the rhombohedral to tetragonal phase transition temperature T_Rho–Tetra = 195 °C, and then a maximum permittivity (36,000 at 10 kHz) at the transition temperature T_m = 277 °C on ceramics prepared with the columbite method. However, piezoelectric coefficient (d₃₃) was measured to be 282 pC/N for the conventional method and higher than the columbite method. The results were related to the phase compositions and porosity of the ceramics.     

Low-temperature preparation and properties of ceramics with composition (1−x)CaTiO3−xPbF2−xLiF

Fluorinated ceramics with initial composition (1−x)CaTiO₃+xPbF₂+xLiF were sintered at 950 °C. The X-ray diffraction (XRD) patterns of the samples showed the formation of a novel solid solution in the initial composition range 0⩽x⩽0.125. SEM observations were performed on fractured ceramics and DSC analyses were carried out from room temperature up to 600 °C. Three second-order phase transitions were detected for all the samples. Capacitors were prepared from the pre-sintered ceramics then dielectric measurements were performed as a function of temperature in the frequency range 10²–4×10⁷ Hz. The ε′_r−T curves exhibit the profile of dielectrics for class I capacitors, however the values of tan δ are too high (tan δ⩾1%).     

Phase evolution and morphology of nanocrystalline BaCe0.9Er0.1O3−δ proton conducting oxide synthesised by a novel modified solution combustion route

Pure BaCeO₃ and 10 mol% Er₂O₃ doped BaCeO₃ (BCE) was synthesised by a novel modified solution combustion synthesis (MCS) route wherein the pH of the precursor solution was varied and the phase formation and morphology were compared with those obtained in conventional solution combustion synthesis (SCS). X-ray diffraction (XRD) studies confirmed the presence of the undesirable BaCO₃ phase in the calcined powders prepared using SCS route whereas the powders synthesised with the modified (MCS) route exhibited a single perovskite phase after calcination. Variation in the pH of the precursor solution resulted in a morphology change from a mix of irregular and globular at pH 4 to more spherical at pH 6 and 8. Fourier transform infrared spectroscopy (FT-IR) studies revealed that calcination time has more pronounced effect on phase formation than calcination temperature. A calcination time of 10 h at 1000 °C resulted in negligible amount of BaCO₃. Such prolonged calcination treatment resulted in substantial grain growth in the SCS sample while the MCS samples were still in the nanocrystalline form. Absence of the ceria peak (464 cm^–1) in the Raman spectra confirmed the presence of a single perovskite BaCeO₃ phase in the sintered pellets as well.     

Superionic transitions in NASICON-type solid electrolytes

Four different methods, temperature dependent X-ray diffraction, impedance measurements, calorimetry and dilatometry, were employed to investigate the superionic transitons of NASICON type materials. Two singularities are found at 160 – 170 °C and about 230 °C. The first one is related to a second order phase transition close to the temperature of structural phase transition; the second one coincides with the inflection in the temperature dependence of conductivity. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–15, 1995     

High-temperature X-ray diffraction and Raman scattering studies of Ba-doped (Na0.5Bi0.5)TiO3 Pb-free piezoceramics

The structural changes in (100 ? x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ (0 ≤ x ≤ 10) ceramics were investigated as a function of composition and temperature by X-ray diffraction and Raman spectroscopy. As Ba concentration increases, the structure changed from rhombohedral to tetragonal (x ≥ 6.5) across a morphotropic phase boundary like phase coexistence at x ～ 5.5, which is further evidenced by phonon anomalies observed in composition-dependent Raman spectra. On heating, the disappearance of peak splits in {111} (x ≤ 5) and {200} (x ≥ 6.5) Bragg peaks and the changes in their 2θ-positions indicated temperature-driven structural changes: ferroelectric to antiferroelectric (≈T_d, depolarization temperature) at 220 °C and antiferroelectric to paraelectric (rhombohedral to tetragonal) at 320 °C. In addition, Raman spectral analysis suggested that at elevated temperatures, two tetragonal phases with slightly different space groups coexisted at x ≥ 6.5 and most of the phase transition temperatures shifted towards left with increasing x.     

The characteristics of Au:VO2 nanocomposite thin film for photo-electricity applications

Au nanoparticles have been fabricated on normal glass substrates using nanosphere lithography (NSL) method. Vanadium dioxide has been deposited on Au/glass by reactive radio frequency (rf) magnetron sputtering. The structure and composition were determined by X-ray diffraction and X-ray photoelectron spectroscope. Electrical and optical properties of bare VO₂ and Au:VO₂ nanocomposite thin films were measured. Typical hysteresis behavior and sharp phase transition were observed. Nanopartical Au could effectively reduce the transition temperature to 40 °C. The transmittance spectrum for both Au:VO₂ nanocomposite thin film shows high transmittance under transition temperature and low transmittance above transition temperature. The characteristics present the Au:VO₂ nanocomposite thin film can be used for applications, such as “smart window” or “laser protector”.     

The phase transition in PrGa0.95Mg0.05O3 at elevated temperatures

PrGa_0.95Mg_0.05O₃ was synthesized by a standard solid-state technique, having an orthorhombic structure determined from X-ray diffraction (XRD) data at room temperature. At high temperature, an inflection is found on the Ln σT vs. 1/T curve at about 550 °C. The inflection can also be observed on thermal expansion vs. temperature curve at about 530 °C. Furthermore, the number of Raman modes between 200 and 400 cm^?1 decreases and the peak at about 190 cm^?1 weakens at about 450 °C. The results indicate a structural transition from orthorhombic to tetrahedral or rhombohedral.     

Study of phase transition of TiO<Subscript>2</Subscript>–CaO system

Electrical conductivity of TiO₂ doped with CaO has been measured at different temperatures for various molar ratios. The conductivity after initially remaining constant till about 140 °C increases with temperature due to the migration of vacancies created by doping. After attaining a maximum value at 240 °C, conductivity decreases due to the collapse of fluorite framework. A second rise in conductivity at high temperature beyond 400 °C indicates the phase transition of TiO₂, from anatase to rutile, which is confirmed by the differential scanning calorimetry results. X-ray powder diffraction, impedance measurements, and Fourier transform infrared spectral studies were also carried out for confirming the doping effect and phase transitions in TiO₂. Doping of TiO₂ with CaO shifts the transition to lower temperatures.     

Effect of co-dopant addition on the properties of yttrium and neodymium doped barium cerate electrolyte

The paper discusses the Y and Nd doped barium cerate perovskites prepared by a modified Pechini method. The series prepared is described by the formula BaCe_0.80Y_xNd_0.2−x O_3−δ. The BaCeO₃ based powders, about 10–20 nm sized and uniformly shaped, were obtained through the calcination of the gel at 500 °C for 2 h. Their structures and ionic conductivities were characterized by X-ray diffraction and AC impedance spectroscopy. All the electrolytes were found to be barium cerate based solid solutions of perovskite type structures. Impedance spectra indicate that the grain boundary resistance of the specimen synthesized by this method is smaller than that of the samples prepared by a conventional solid-state method. The ionic conductivities of co-doped barium cerate were a factor of several times than that of the single-doped one at 673–1073 K. Among the electrolytes examined, the one co-doped with 5 mol% Nd and 15 mol% Y shows the best improvement in performance. These co-doped barium cerate are more ideal intermediate temperature (IT) electrolyte materials.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

Characterization of Cd1−xFexS diluted magnetic semiconductors grown at near phase conversion temperature

Fe-based cadmium sulfide alloy thin films have been grown on c-plane sapphire substrates by a low-pressure metalorganic chemical vapor deposition technique at different growth temperatures. From X-ray diffraction and absorption spectra of the samples, the evolutions with growth temperature show an inflexion at the growth temperature of 300 ^°C. This was attributed to the phase transformation from zinc-blende to wurtzite. With increasing growth temperature from 270 ^°C to 360 ^°C, Fe concentration in the films increases monotonously. The electronic states of Cd_1−xFe_xS were investigated by X-ray photoelectron spectroscopy. Magnetic measurement shows Van Vleck paramagnetism of the Cd_1−xFe_xS thin film in the temperature region below 7 K.     

Structural,optical, and electrical properties of tin sulfide thin films grown with electron-beam evaporation

The effect of growth temperature on the phase evolution and morphology change of tin sulfide thin films by electron-beam evaporation was investigated. Orthorhombic tin monosulfide (SnS) was dominant at low growth temperature of 25 °C, whereas a sulfur-rich phase of Sn₂S₃ coalesced as the growth temperature increased over 200 °C. Thin film growth ceased at 280 °C due to re-evaporation of the tin sulfide. The dependence of growth temperature on the phase evolution of tin sulfide was confirmed by X-ray diffraction, scanning electron microscopy, and UV–Vis spectrophotometry. The lowest electrical resistivity of ∼51 Ω cm, with a majority hole concentration of ∼10¹⁷ cm⁻³, was obtained for the film grown at 100 °C, and the resistivity drastically increased with increasing growth temperature. This behavior was correlated with the emergence of resistive sulfur-rich Sn₂S₃ phase at high temperatures.     

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Synthesis,deposition and characterization of tin selenide thin films by thermal evaporation technique

Tin selenide alloy was synthesized by following simple chemical reaction method, at comparatively lower temperature of 100 °C, from alkaline medium using SnCl₂.2H₂O and selenium as source materials. Powder X-ray diffraction analysis reveals that the particle size of the synthesized product is in nanometer scale. Using the reaction product as source material, the SnSe films were deposited on glass substrates at room temperature, 150 °C, 250 °C, 350 °C and 450 °C. Structural, elemental, optical, surface morphological and electrical properties of the as deposited films were studied by X-ray diffraction, Energy Dispersive X-ray Analysis, UV-Vis-NIR, Scanning Electron Microscopy and Hall effect measurement techniques and the relevant details have been obtained.     

Kinetic study of the thermal transformation of limonite to hematite by X-ray diffraction, ��-Raman and M?ssbauer spectroscopy

A kinetic study about the phase limonite (FeO(OH)-nH₂O) was performed through X-ray diffraction, ??-Raman spectroscopy and Mössbauer spectroscopy. The oxide powder sample was extracted from Taraco district, Huancané province of Puno (Peru). X-ray diffraction identified the phase goethite as the main mineralogical component, and then the sample was subjected to in-situ heat treatment in the temperature range: 100 to 500°C in oxidizing (air) and inert (nitrogen) atmospheres. The goethite phase remains stable in this range: room temperature to 200°C. Between 200°C to 250°C there is a phase transition: ??-Fe^3?+?O(OH) ?? ??-Fe₂O₃, i.e., from goethite to hematite phase, taking as evidence the evolution of the diffraction profiles. At 200°C spectra shows the start of broadened magnetic component and it was adjusted through of a magnetic distribution giving a mean field of 38.6T and a relative area of 52.9%, which is a characteristic of goethite. Also, it is noticed the presence of a small amount of hematite with a mean field of 49.0T linked with a superparamagnetic broadened doublet of relative area of 47.1% where the domains of the particles have sizes smaller than 100 Å and it is evidence the superparamagnetic limit; i.e., the superparamagnetic effect tends toward a distribution of magnetic fields. Moreover, the Raman spectra of the in-situ thermal treatment, support the transition at 290°C through the transformation of characteristic bands of goethite to hematite phase at the frequency range from 200 to 1,800 cm^???1.     

Impedance spectroscopy study of the sintering of yttria-stabilized zirconia/magnesia composites

In the present study, the sintering of (ZrO₂:8 mol%Y₂O₃)_1 ? m–(MgO)_m, YSZ–mMg composites, with m in the 0–30 mol% range, has been investigated by impedance spectroscopy (IS), dilatometry, and X-ray diffraction. Impedance diagrams were collected at 400 °C after heating the green compacts up to a selected sintering temperature, which was increased stepwise from 800 to 1400 °C. The combined experimental results revealed that the samples can be separated in two categories: below and above the solubility limit of MgO in the YSZ (m ~ 10). Moreover, important microstructural features associated with both the sintering process and solid solution formation of YSZ–mMgO samples were correlated to the electrical properties inferred by IS.     

Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1−xCoxO3−δ materials

Oxygen deficiency, thermal and chemical expansion of La_0.5Sr_0.5Fe_1−xCo_xO_3−δ (x = 0, 0.5, 1) have been measured by thermogravimetry, dilatometry and high temperature X-ray diffraction. The rhombohedral perovskite materials transformed to a cubic structure at 350 ± 50 °C. The thermal expansion of the materials up to the onset of thermal reduction was 14–18 × 10^− 6 K^− 1. Above 500 °C in air (400 °C in N₂), chemical expansion contributed to the thermal expansion and the linear thermal expansion coefficients were significantly higher, 16–35 × 10^− 6 K^− 1. The chemical expansion, ε_c, showed a maximum of 0.0045 for x = 0.5 and 0.0041 for x = 1 at 800–900 °C. The normalized chemical expansion, ε_c/Δδ, was 0.036 for x = 0.5 and 0.035 for x = 1 at 800 °C. The chemical expansion can be correlated with an increasing ionic radius of the transition metals with decreasing valence state.     

Acoustic anomalies and relaxation dynamics of relaxor ferroelectric Na1/2Bi1/2TiO3 single crystals studied by using Brillouin spectroscopy

Acoustic anomalies of relaxor ferroelectric Na_1/2Bi_1/2TiO₃ single crystals were investigated over a wide temperature range from −196 °C to 900 °C by using Brillouin spectroscopy. The longitudinal sound velocity, the acoustic absorption coefficient and the elastic constant C₁₁ were determined for the acoustic phonon mode propagating in the [100] direction. Two acoustic anomalies, weaker ones at the cubic-tetragonal phase transition temperature of ~540 °C and more pronounced ones at temperatures near 315 °C near the dielectric maximum temperature, were investigated and discussed in relation with the relevant order parameters coupled to the acoustic waves. The relaxation dynamics in the cubic phase were studied based on the flattening of the mode frequency and the half width, which was observed for the first time, and a modified Arrhenius law.